Field of the Invention
The present invention relates to a low refractive index film-forming composition for forming a low refractive index film which is used for a display panel, a solar cell, an optical lens, a camera module, a sensor module, or the like; and a method of forming a low refractive index film. More specifically, the invention relates to a low refractive index film-forming composition for forming an antireflection film which prevents the reflection of incident light in the solar cell and the like or for forming an interlayer film which is used for a sensor, a camera module, or the like and uses a refractive index difference; and a method of forming a low refractive index film using the same.
Priority is claimed on Japanese Patent Application No. 2012-128553, filed on Jun. 6, 2012, the content of which is incorporated herein by reference.
Description of Related Art
A film having a low refractive index which is formed on a surface of a transparent substrate such as glass or plastic is used as an antireflection film for preventing the reflection of incident light on a display panel such as a cathode-ray tube display, a liquid crystal display, or an organic EL display, solar cells, an optical lens, or a showcase glass. For example, an antireflection film for improving visibility is provided on a display surface side of a display panel. In addition, in the field of a solar cell, in order to prevent the reflection of incident sunlight and to improve the absorptance of light, a countermeasure is provided, for example, a film having a low refractive index is formed on a surface or the like of a glass substrate as an antireflection film.
As such a film for preventing the reflection, in the related art, a single-layer film which is formed using a gas phase method such as a vacuum deposition method, a sputtering method, or the like and is formed of MgF2, cryolite, or the like, has been put into practice. In addition, a multi-layer film or the like, which is formed by alternately laminating a low refractive index film formed of SiO2 or the like and a high refractive index film formed of TiO2, ZrO2, or the like on a substrate, is also known to be able to obtain a high antireflection effect. However, in the gas phase method such as a vacuum deposition method, a sputtering method, or the like, there is a problem in terms of manufacturing cost because a manufacturing apparatus and the like are expensive. In addition, in the method of forming a multi-layer film by alternately laminating a low refractive index film and a high refractive index film, the manufacturing process is complicated and much time and effort are required, which is not practical.
Therefore, from the viewpoint of manufacturing cost or the like, recently, a coating method such as a sol-gel method has attracted much attention. In the sol-gel method, generally, a sol-gel solution is prepared and a transparent substrate such as glass is coated with this solution, followed by drying, firing, and the like to form a film. However, the film formed using the sol-gel method has various problems in that a desired low refractive index is not obtained; and that defects of adhesion with the substrate and cracks are generated, as compared to the film formed using the gas phase method such as a vacuum deposition method.
As such a film forming method using the sol-gel method, a method of forming a coating film is disclosed in which a silicon compound (A) which is tetraalkoxysilane, a silicon compound (B) which is fluorine-containing trialkoxysilane, alcohol (C), and oxalic acid (D) are mixed with each other at a predetermined ratio to obtain a reaction mixture; the reaction mixture is heated at a predetermined temperature while maintaining the absence of water to prepare a polysiloxane solution; and a surface of a substrate is coated with this polysiloxane solution, followed by thermal curing (for example, refer to Japanese Unexamined Patent Application First Publication No. H9-208898 (claim 1 and paragraph [0009] to paragraph [0010])).
In this method, the polysiloxane solution is prepared not by condensation using hydrolysis of the silicon compound (A) and the silicon compound (B) but by heating in the reaction mixture in the absence of water. As a result, during the reaction, the polysiloxane solution does not become cloudy and the inhomogeneous production of polysiloxane can be suppressed. In addition, even when a ratio of the silicon compound (B) is reduced compared to the related art, a film having a low reflective index can be obtained.